Merge branch 'cf/c23-const-preserving-strchr-updates-0'

ISO C23 redefines strchr and friends that tradiotionally took
a const pointer and returned a non-const pointer derived from it to
preserve constness (i.e., if you ask for a substring in a const
string, you get a const pointer to the substring).  Update code
paths that used non-const pointer to receive their results that did
not have to be non-const to adjust.

* cf/c23-const-preserving-strchr-updates-0:
  gpg-interface: remove an unnecessary NULL initialization
  global: constify some pointers that are not written to

diff --cc add-patch.c
Simple merge

diff --cc builtin/receive-pack.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc help.c
Simple merge

diff --cc string-list.c
Simple merge